The invention involves the acquisition and processing of three dimensional (3D) seismic data, and, in particular, the acquisition and processing of seismic data using a sampling grid.
Three dimensional seismic exploration of the earth's terrestrial and marine subsurface is used in the exploration, production, and development of hydrocarbons. In seismic exploration, underground geological structures are illuminated and imaged with sound energy. The basic components of a surface seismic exploration system would include a source (e.g., a dynamite explosion or airgun sounding) on the earth's surface and a receiver on the surface. The source generates sound waves to be reflected from a subsurface structure and the receiver receives and records the upwardly reflected sound energy. Data from the seismic experiment are recorded in the form of a seismic trace, i.e., a record of the reflected sound events received at the surface receiver over a period of time. The image of the subsurface is constructed by processing the reflected energy from many of these seismic traces.
In "prestack imaging," seismic traces from single source-receiver pairs are processed individually. In "poststack imaging," on the other hand, data from many single source-receiver pairs are combined by stacking (summing) before processing to increase the signal-to-noise ratio and to reduce the number of seismic traces that must be processed. The traces are first grouped into "common midpoint gathers" and then operations such as Normal Movement ("NMO") and Dip Moveout ("DMO") are performed. A midpoint is defined as the surface location halfway between a trace's source and receiver.
After NMO and DMO stacking, source and receiver locations are considered identical and equal to the midpoint location. However, because of obstructions, irregular topography, feathered cables at sea, etc., even the most careful planning for recording groups of traces generally does not result in traces which share exactly the same midpoint. To accommodate these midpoint variations, 3D seismic exploration uses a layout of sources and receivers in a regular geometric configuration. For example, a regular two-dimensional grid, traditionally rectangular or square, is superimposed on the earth's surface. The midpoints between each source and receiver pair are assigned to the nearest grid point and surrounding cell or bin. This process is termed "binning". With binning, a piece of data with an analog position (i.e., acquired at an analog position in a coordinate system) is assigned to the nearest point on a sampling raster or regular array (e.g., to a gridpoint or a grid cell center point) that can be processed by a digital computer. The size of the grid cell is determined by the spatial sampling criteria in such a way that the geological objectives of the survey are met. Bin size is determined by the source spacing and receiver spacing (interval between consecutive sources and consecutive receivers). There is an inverse relation between the bin size and the cost of the survey for acquisition and processing.
An example layout of sources (source stations) 124 and receivers (receiver stations) 108 in a regular geometric configuration is shown in FIG. 1A. Using such a configuration, three-dimensional acquisition systems for seismic exploration (cable surveys) on land, in transition zones (areas between land and deep water), and in the sea are similar. One or more receiver cable lines 102 and one or more source cable lines 112 are usually used to form the source and receiver layout. The source lines 112 may be perpendicular to the receiver lines 102, or they may be parallel to, or at a particular diagonal angle to, the receiver lines 102 (not shown).
FIG. 1B shows a regular grid 111 (e.g., a rectangular or square grid) superimposed on the regular geometric configuration of the sources 124 and receivers 108 of FIG. 1A. The regular grid 111 defines individual bins 114.
For example, the 3D acquisition geometry provided by the configuration of FIG. 1A is determined by the following setup or known parameters: receiver station 108 (-to-station 108 distance) interval, source station 124 (-to-station 124 distance) interval, receiver line 102 (-to-line 102 distance) interval, source line 112 (-to-line 112 distance) interval, and receiver spread (number of receiver lines 102 and receiver stations 108 active at any given time). These five parameters depend on: bin size, maximum offset (the maximum distance between a source and an active receiver), minimum offset, fold (how many samples are going to be put into a bin), offset distribution (e.g., mostly near or mostly far) and azimuth distribution (e.g., all sources and receivers are lined up along a north and south direction or for different angles through the bins).
The bin size of the bins 114 defined in FIG. 1B (referred to by the spacings, ".DELTA.x.sub.bin ", ".DELTA.y.sub.bin ") in both the perpendicular x and y directions is dictated by the spatial sampling requirements that are functions of the seismic resolution requested or needed to properly interpret the seismic data.
As shown in FIG. 1B, the receiver station 108 spacing interval, .DELTA.r, and the bin 114 spacing in the receiver line 102 (see FIG. 1A) direction, .DELTA.x.sub.bin, may be related by: .DELTA.x.sub.bin =.DELTA.r/2. The source station 124 interval, .DELTA.s, and the bin 114 size in the source line 112 (see FIG. 1A) direction, .DELTA.y.sub.bin, may be related by: .DELTA.y.sub.bin =.DELTA.s/2. With grid 111 and these relationships between .DELTA.x.sub.bin and .DELTA.r and .DELTA.y.sub.bin and .DELTA.s, the locations of the midpoints between each source 124 and receiver 108 will be at the center of bins 114, for example, at bin center gridpoints 110 in FIG. 1B (not all bin center gridpoints 110 are shown in FIG. 1B).